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Abstract The Newcastle disease is an acute infectious disease， which spreads rapidly and causes serious damage to the chicken industry. In recent years， due to various reasons， atypical， chronic Newcastle disease has appeared in several chicken farms， especially in the process of stocking in mountain forests. In this paper， the author analyzed the causes of atypical Newcastle disease occurred in mountain forest stocking chickens from a farm in Zhongfang County， Huaihua City， and proposed some control measures.
Key words Mountain forest chicken; Atypical Newcastle disease; Huaihua; Diagnosis; Analysis
Newcastle disease is an acute， febrile， septic and highly contagious disease caused by Newcastle disease virus in chickens， which is characterized by high fever， dyspnea， diarrhea， neurological disorders， mucosal and serosal bleeding with extremely high incidence and mortality， thus endangering the chicken industry[1]. However， due to various reasons， Newcastle disease appears commonly in an atypical form and exhibits no typical symptoms in clinical practice， which may delay treatment because it cannot be diagnosed in time and accurately， thereby bringing great harm to the breeding industry. Previously， we encountered a case of atypical Newcastle disease. Immunized chickens have no typical characteristics of Newcastle disease， and the symptoms are not severe. Young chickens exhibit mild respiratory symptoms and movement disorders， accompanied by sporadic deaths， resulting in unnecessary losses.
Causes and Clinical Symptoms
In a chicken farm in Zhongfang County， Huaihua City， four thousand 43dayold green shell hens were afflicted， all of which were immunized with a vaccine against infectious bursal disease and a vaccine against Newcastle disease twice. Clinical symptoms mainly occurred in respiratory tract and digestive tract， including a slight decrease in appetite， mouth opening and neck stretching， wheezing， difficulty in breathing， dryness and dullness of the skin and claws. Yellowish green loose stools were excreted at the time of onset， and yellow stools with greenish mucous purulent substances were excreted at the late stage， accompanied by sporadic deaths， about 1-5 hens per day. Treatment with florfenicol and amoxicillin had no obvious effects or even turned to be invalid.
Pathological Changes
Seven infected chickens were dissected. Grayish white or yellowwhite mucus was found in the trachea. There were bleeding points in the throat and the upper part of the trachea， especially in the bifurcation of the trachea and bronchi. A layer of grayish white viscous secretions formed on the glandular mucosa. Bleeding of glandular stomach teats was rare （2/7）. Purulent secretions sometimes expelled from glandular stomach teats after squeezing （4/7）. The lymph nodes in the duodenum and yolk pedicle were aggregated and exhibited swelling or bleeding. There was obvious hemorrhage in the posterior segment of the rectum. The cloaca mucosa showed brushlike hemorrhage （typical Newcastle disease resulted in diffuse hemorrhage） （3/7）. The cecal tonsils exhibited swelling and bleeding （2/7）. There were protrusions with hemorrhage in the ileum， containing chicken cestodes （2/7）. Diagnosis
According to the epidemiological characteristics， clinical symptoms and pathological changes， a preliminary diagnosis could be conducted for indepth laboratory diagnosis. The bacteria culture was performed at 37 ℃ for 24 h. Microscopic examination showed that there were no bacteria. By on hemagglutination inhibition test， it was confirmed that the chickens were infected with atypical Newcastle disease.
Cause Analysis
Use of antibiotics and antiviral drugs
Various antibiotics and antiviral drugs have different immunosuppressive effects， of which antiviral drugs have much greater effects. The use of these drugs before and after immunization can cause immunosuppression， thus affecting the immune effect of vaccines. Therefore， it is necessary to avoid the use of antibiotics and prohibit the use of antiviral drugs before and after immunization[2].
Vaccines
Excessive temperature or repeated freezing and thawing may result in the reduction or loss of vaccine effect during transportation or storage. In addition， during the dilution of the vaccine， inaccurate calculation or weighing may increase the amount of the diluent， resulting in insufficient virus strains per unit vaccine. Diluting the vaccine in advance and preserving the diluted vaccine at room temperature before inoculation may reduce vaccine titers for later use[3].
Improper immunization
Improper immunization programs may leads to the occurrence of atypical Newcastle disease. After immunization in advance， the vaccine can not break through the maternal barrier and may be quickly neutralized， failing in exerting the preventive effect. In addition， simultaneous use of several vaccines or short interval between the use of different vaccines may also cause immune interference.
Each vaccine has its best immune pathway. The immunity against Newcastle disease includes humoral immunity and local mucosal immunity of respiratory tract， of which the previous respiratory mucosal immunity is much more important. In farms with large populations， the breeders generally focus on drinking immunization while ignoring local immunization. The Harders gland below the iris has important significance to the mucosal immunity at the early stage. Therefore， it is necessary to perform eye dropping during initial immunization[4].
Poor feeding management
In order to reduce the cost of raising， mountain forest chickens are mostly cultivated in greenhouses， which are simple with poor heat preservation and ventilation capacity and bad environment， thus reducing the resistance of the chickens. Once the epidemic prevention and disinfection measures are weak， it is easy to cause occurrence and prevalence of Newcastle disease. Although some farms have performed immunization strictly according to the immunization procedures and requirements， they may neglect epidemic prevention measures such as disinfection and isolation and of sick chickens. Randomly discarding infected and dead chickens or selling sick chickens to reduce economic losses may cause the spread of Newcastle disease to a wider extent. Poor feeding management of chicken farms will cause subhealth of some chickens. There may be a strong stress reaction during immunization， inducing symptoms of some latently infected diseases， so that the immunization will not achieve the expected results. Under poor feeding management conditions， the bad environment and stress hazard factors （such as inappropriate temperature and humidity of the chicken houses， high concentrations of NH3， H2S and SO2 due to bad ventilation， large stocking density） will decline the immune function. In addition， sudden changes in temperature， feed ingredients and feeding patterns， noise， debeaking， and group transfer may induce severe stress reactions in the flock and reduce immunity， thereby enhancing the possibility of NDV infection[5].
Interference with immunosuppressive diseases
There are many diseases in chicken farms that can cause immunosuppression in chickens， which seriously affects the immune effect against Newcastle disease or even cause immune failure. Subclinical infection of infectious bursal disease， chicken infectious anemia， reticuloendotheliosis， Mareks disease， avian leukemia， coccidiosis and aflatoxin poisoning will damage the immune organs and cause immunosuppression. After immunization， chickens can not produce normal immune responses to reach the corresponding antibody level， which may be easily infected with NDV[6].
In recent years， chicken farms have increasingly recognized the importance of immunization. In the case of high antibody titers with good uniformity in the body， excessive immunization of attenuated vaccines will， on the contrary， easily reduce antibody levels. Chickens can be easily infected with NDV when antibodies are lowered. In addition， frequent stimulation of the antigen will cause fatigue responses. The original antibodies can still protect the body from NDV infection. After immunization， the vaccine first neutralizes some antibodies， so that the antibody level drops sharply in a short time and chickens may be easily infected with NDV， resulting in immunosuppression. Therefore， the chickens may remain healthy without immunization， while being infected three or two days after immunization.
Lack of nutrients in feed
The lack of nutrients such as proteins， vitamins and minerals in the feed may affect the immune function of the body to different degrees， so that chickens will not produce normal immune response during normal immunization， thus reducing the immune effect. Control Measures
Correct selection of highquality vaccines
It is necessary to ensure high quality， good preservation and correct use of the vaccine to improve the immune effect. First of all， vaccines should be purchased from manufacturers approved by the national qualification with high credibility and good reputation among the farmers. The production batch number， production date and other instructions of the vaccine should be checked carefully to understand the transportation and storage conditions of the vaccine.
The purchased vaccines should be stored as required to avoid high temperature and direct sunlight. Dilution should be performed according to the instructions. The attenuated vaccines should be placed in a cool place and should be used preferably within 4 hours; the inactivated vaccines should be used within 24 hours after opening. Whether to stop using antibiotics before and after inoculation of attenuated vaccines must be determined based on the epidemic situation， and it is not necessary to stop using antibiotics in all cases. In addition， when bacterial diseases occur， the chickens are immunized in the morning and should be treated with drugs in the afternoon to control the bacterial diseases. Drugs that affect the vitality of the vaccine and the immune response should be avoided. When the farms pay attention to the above problems， the occurrence of atypical Newcastle disease will be reduced greatly.
Strengthening feeding management
A single species of poultry should be raised in the farm， while avoiding raising a variety of breeds in the farm， in order to prevent mutual infection and cut off the source of the disease. In the breeding process， isolation and regular disinfection should be conducted. In addition， mutual flow of different products should be avoided by demarcating the production area and living area in the farm. It is necessary to improve the breeding environment （such as reasonable stocking density， suitable temperature， good ventilation， fresh air）， provide full price feed to guarantee balance of various nutrients （methionine， valine， selenium， vitamins A， E and C） in the feed and meet nutritional needs， and reduce stress responses by adding some antistress drugs， such as vitamins C and E， multiplex vitamin of electrolyte， and glucose.
Infection with some viral diseases （such as AI， IBD， MD， IB） and bacterial diseases （such as chicken pullorum， colibacillosis） will also affect the health status and immune effect of the flock. Therefore， it is necessary to prevent secondary infection and mixed infection of bacterial diseases and viral diseases， so as to control the occurrence of Newcastle disease. Avoiding immunosuppressive reactions
Infected chickens can not be immunized. Chickens infected with infectious bursal disease， Mareks disease and aflatoxin poisoning are temporarily unable to be injected. Otherwise， the immunity will be suppressed. Moreover， immunization of chickens that are infected with avian cholera may cause deaths of a large number of chickens.
It is necessary to avoid using drugs that can cause immunosuppression during immunization. Living disinfection should not be performed a few days before and after immunization. Electrolytes and multivitamins can be added into the drinking water to increase antistress ability of the body.
Development and implementation of reasonable immunization programs
Appropriate immune procedures and immune pathways should be developed according to the day age， vaccine properties and local prevalence of diseases. A reasonable immunization program should be established based on local epidemic situation， the growth and decline patterns of antibodies， and the interference relationship between vaccines. Qualified farms and households should detect antibodies regularly to determine the time of immunization， so as to improve the uniformity of the immune response and avoid "immune blanking period" and "immune paralysis"[7].
To overcome the deficiency of drinking immunization， the vaccine can be administered orally by drinking so as to improve the immune effect. Thus， all chickens can obtain enough vaccine through drinking in a short time. Specifically， the water is supplied for 2-3 hours， and the vaccine is added twice into the drinking water; the drinking was uninterrupted and lasted 1 hour each time. The vaccine should be prepared immediately before use to ensure a good immune effect.
The immunization， detection and purification of chickens should be strengthened. In addition， it is necessary to adhere to regular immune monitoring. The critical point of antibody level against Newcastle disease is 3-4 titers. The antibody level higher than 3 log2 can only ensure that there are no massive deaths in clinical practice. However， when the antibody level is lower than 7 titers， the amount of eggs produced will be greatly reduced during the invasion of a strong virus strain. Therefore， HI antibody level of the breeder should be higher than 7 titers.
Feeding highquality fullprice feed to ensure good body constitution
Some farms feed the chickens with "lowprice feed"， which may reduce the resistance of chickens. Therefore， the breeders should establish the idea of "high input and high output" and feed the chickens with good quality feed to enhance body constitution and improve disease resistance. Wuzhen SU et al. Diagnosis and Etiological Analysis of Atypical Newcastle Disease in Mountain Forest Chickens in Huaihua City
References
[1] LI SS， LI JY， et al. Analysis of the causes of local epidemic of chicken Newcastle disease and its countermeasures[J]. Guide to Chinese Poultry， 2007 （1）：30.（in Chinese）
[2] WU D， GAO GZ， et al. Analysis of the causes and prevention of atypical Newcastle disease in hens[J]. Veterinary Orientation， 2007 （9）：27-28.（in Chinese）
[3] LU MY. Prevention of atypical Newcastle disease in chickens[J]. Sichuan Animal & Veterinary Sciences， 2007 （9）：56-57.（in Chinese）
[4] WANG RD， BAI J， MA HL. Epidemiology and preventing measure of Newcastle disease[J]. China Animal Husbandry & Veterinary Medicine， 2005 （4）： 60-62.（in Chinese）
[5] ZHOU RH， DING Y， et al. Cause analysis and prevention measures of immune failure against Newcastle disease[J]. Livestock and Poultry Industry， 2007 （6）： 37-38.（in Chinese）
[6] HU H. Characteristics of occurrence and prevalence of poultry diseases in recent year in Huaihua City， Hunan Province[J]. Guide to Chinese Poultry， 2006， 23（11）：24.（in Chinese）
[7] HAN DQ， WANG H. Diagnosis and prevention of Newcastle disease in blackbone chickens[J]. China Poultry， 2006 （10）：49.（in Chinese）
Key words Mountain forest chicken; Atypical Newcastle disease; Huaihua; Diagnosis; Analysis
Newcastle disease is an acute， febrile， septic and highly contagious disease caused by Newcastle disease virus in chickens， which is characterized by high fever， dyspnea， diarrhea， neurological disorders， mucosal and serosal bleeding with extremely high incidence and mortality， thus endangering the chicken industry[1]. However， due to various reasons， Newcastle disease appears commonly in an atypical form and exhibits no typical symptoms in clinical practice， which may delay treatment because it cannot be diagnosed in time and accurately， thereby bringing great harm to the breeding industry. Previously， we encountered a case of atypical Newcastle disease. Immunized chickens have no typical characteristics of Newcastle disease， and the symptoms are not severe. Young chickens exhibit mild respiratory symptoms and movement disorders， accompanied by sporadic deaths， resulting in unnecessary losses.
Causes and Clinical Symptoms
In a chicken farm in Zhongfang County， Huaihua City， four thousand 43dayold green shell hens were afflicted， all of which were immunized with a vaccine against infectious bursal disease and a vaccine against Newcastle disease twice. Clinical symptoms mainly occurred in respiratory tract and digestive tract， including a slight decrease in appetite， mouth opening and neck stretching， wheezing， difficulty in breathing， dryness and dullness of the skin and claws. Yellowish green loose stools were excreted at the time of onset， and yellow stools with greenish mucous purulent substances were excreted at the late stage， accompanied by sporadic deaths， about 1-5 hens per day. Treatment with florfenicol and amoxicillin had no obvious effects or even turned to be invalid.
Pathological Changes
Seven infected chickens were dissected. Grayish white or yellowwhite mucus was found in the trachea. There were bleeding points in the throat and the upper part of the trachea， especially in the bifurcation of the trachea and bronchi. A layer of grayish white viscous secretions formed on the glandular mucosa. Bleeding of glandular stomach teats was rare （2/7）. Purulent secretions sometimes expelled from glandular stomach teats after squeezing （4/7）. The lymph nodes in the duodenum and yolk pedicle were aggregated and exhibited swelling or bleeding. There was obvious hemorrhage in the posterior segment of the rectum. The cloaca mucosa showed brushlike hemorrhage （typical Newcastle disease resulted in diffuse hemorrhage） （3/7）. The cecal tonsils exhibited swelling and bleeding （2/7）. There were protrusions with hemorrhage in the ileum， containing chicken cestodes （2/7）. Diagnosis
According to the epidemiological characteristics， clinical symptoms and pathological changes， a preliminary diagnosis could be conducted for indepth laboratory diagnosis. The bacteria culture was performed at 37 ℃ for 24 h. Microscopic examination showed that there were no bacteria. By on hemagglutination inhibition test， it was confirmed that the chickens were infected with atypical Newcastle disease.
Cause Analysis
Use of antibiotics and antiviral drugs
Various antibiotics and antiviral drugs have different immunosuppressive effects， of which antiviral drugs have much greater effects. The use of these drugs before and after immunization can cause immunosuppression， thus affecting the immune effect of vaccines. Therefore， it is necessary to avoid the use of antibiotics and prohibit the use of antiviral drugs before and after immunization[2].
Vaccines
Excessive temperature or repeated freezing and thawing may result in the reduction or loss of vaccine effect during transportation or storage. In addition， during the dilution of the vaccine， inaccurate calculation or weighing may increase the amount of the diluent， resulting in insufficient virus strains per unit vaccine. Diluting the vaccine in advance and preserving the diluted vaccine at room temperature before inoculation may reduce vaccine titers for later use[3].
Improper immunization
Improper immunization programs may leads to the occurrence of atypical Newcastle disease. After immunization in advance， the vaccine can not break through the maternal barrier and may be quickly neutralized， failing in exerting the preventive effect. In addition， simultaneous use of several vaccines or short interval between the use of different vaccines may also cause immune interference.
Each vaccine has its best immune pathway. The immunity against Newcastle disease includes humoral immunity and local mucosal immunity of respiratory tract， of which the previous respiratory mucosal immunity is much more important. In farms with large populations， the breeders generally focus on drinking immunization while ignoring local immunization. The Harders gland below the iris has important significance to the mucosal immunity at the early stage. Therefore， it is necessary to perform eye dropping during initial immunization[4].
Poor feeding management
In order to reduce the cost of raising， mountain forest chickens are mostly cultivated in greenhouses， which are simple with poor heat preservation and ventilation capacity and bad environment， thus reducing the resistance of the chickens. Once the epidemic prevention and disinfection measures are weak， it is easy to cause occurrence and prevalence of Newcastle disease. Although some farms have performed immunization strictly according to the immunization procedures and requirements， they may neglect epidemic prevention measures such as disinfection and isolation and of sick chickens. Randomly discarding infected and dead chickens or selling sick chickens to reduce economic losses may cause the spread of Newcastle disease to a wider extent. Poor feeding management of chicken farms will cause subhealth of some chickens. There may be a strong stress reaction during immunization， inducing symptoms of some latently infected diseases， so that the immunization will not achieve the expected results. Under poor feeding management conditions， the bad environment and stress hazard factors （such as inappropriate temperature and humidity of the chicken houses， high concentrations of NH3， H2S and SO2 due to bad ventilation， large stocking density） will decline the immune function. In addition， sudden changes in temperature， feed ingredients and feeding patterns， noise， debeaking， and group transfer may induce severe stress reactions in the flock and reduce immunity， thereby enhancing the possibility of NDV infection[5].
Interference with immunosuppressive diseases
There are many diseases in chicken farms that can cause immunosuppression in chickens， which seriously affects the immune effect against Newcastle disease or even cause immune failure. Subclinical infection of infectious bursal disease， chicken infectious anemia， reticuloendotheliosis， Mareks disease， avian leukemia， coccidiosis and aflatoxin poisoning will damage the immune organs and cause immunosuppression. After immunization， chickens can not produce normal immune responses to reach the corresponding antibody level， which may be easily infected with NDV[6].
In recent years， chicken farms have increasingly recognized the importance of immunization. In the case of high antibody titers with good uniformity in the body， excessive immunization of attenuated vaccines will， on the contrary， easily reduce antibody levels. Chickens can be easily infected with NDV when antibodies are lowered. In addition， frequent stimulation of the antigen will cause fatigue responses. The original antibodies can still protect the body from NDV infection. After immunization， the vaccine first neutralizes some antibodies， so that the antibody level drops sharply in a short time and chickens may be easily infected with NDV， resulting in immunosuppression. Therefore， the chickens may remain healthy without immunization， while being infected three or two days after immunization.
Lack of nutrients in feed
The lack of nutrients such as proteins， vitamins and minerals in the feed may affect the immune function of the body to different degrees， so that chickens will not produce normal immune response during normal immunization， thus reducing the immune effect. Control Measures
Correct selection of highquality vaccines
It is necessary to ensure high quality， good preservation and correct use of the vaccine to improve the immune effect. First of all， vaccines should be purchased from manufacturers approved by the national qualification with high credibility and good reputation among the farmers. The production batch number， production date and other instructions of the vaccine should be checked carefully to understand the transportation and storage conditions of the vaccine.
The purchased vaccines should be stored as required to avoid high temperature and direct sunlight. Dilution should be performed according to the instructions. The attenuated vaccines should be placed in a cool place and should be used preferably within 4 hours; the inactivated vaccines should be used within 24 hours after opening. Whether to stop using antibiotics before and after inoculation of attenuated vaccines must be determined based on the epidemic situation， and it is not necessary to stop using antibiotics in all cases. In addition， when bacterial diseases occur， the chickens are immunized in the morning and should be treated with drugs in the afternoon to control the bacterial diseases. Drugs that affect the vitality of the vaccine and the immune response should be avoided. When the farms pay attention to the above problems， the occurrence of atypical Newcastle disease will be reduced greatly.
Strengthening feeding management
A single species of poultry should be raised in the farm， while avoiding raising a variety of breeds in the farm， in order to prevent mutual infection and cut off the source of the disease. In the breeding process， isolation and regular disinfection should be conducted. In addition， mutual flow of different products should be avoided by demarcating the production area and living area in the farm. It is necessary to improve the breeding environment （such as reasonable stocking density， suitable temperature， good ventilation， fresh air）， provide full price feed to guarantee balance of various nutrients （methionine， valine， selenium， vitamins A， E and C） in the feed and meet nutritional needs， and reduce stress responses by adding some antistress drugs， such as vitamins C and E， multiplex vitamin of electrolyte， and glucose.
Infection with some viral diseases （such as AI， IBD， MD， IB） and bacterial diseases （such as chicken pullorum， colibacillosis） will also affect the health status and immune effect of the flock. Therefore， it is necessary to prevent secondary infection and mixed infection of bacterial diseases and viral diseases， so as to control the occurrence of Newcastle disease. Avoiding immunosuppressive reactions
Infected chickens can not be immunized. Chickens infected with infectious bursal disease， Mareks disease and aflatoxin poisoning are temporarily unable to be injected. Otherwise， the immunity will be suppressed. Moreover， immunization of chickens that are infected with avian cholera may cause deaths of a large number of chickens.
It is necessary to avoid using drugs that can cause immunosuppression during immunization. Living disinfection should not be performed a few days before and after immunization. Electrolytes and multivitamins can be added into the drinking water to increase antistress ability of the body.
Development and implementation of reasonable immunization programs
Appropriate immune procedures and immune pathways should be developed according to the day age， vaccine properties and local prevalence of diseases. A reasonable immunization program should be established based on local epidemic situation， the growth and decline patterns of antibodies， and the interference relationship between vaccines. Qualified farms and households should detect antibodies regularly to determine the time of immunization， so as to improve the uniformity of the immune response and avoid "immune blanking period" and "immune paralysis"[7].
To overcome the deficiency of drinking immunization， the vaccine can be administered orally by drinking so as to improve the immune effect. Thus， all chickens can obtain enough vaccine through drinking in a short time. Specifically， the water is supplied for 2-3 hours， and the vaccine is added twice into the drinking water; the drinking was uninterrupted and lasted 1 hour each time. The vaccine should be prepared immediately before use to ensure a good immune effect.
The immunization， detection and purification of chickens should be strengthened. In addition， it is necessary to adhere to regular immune monitoring. The critical point of antibody level against Newcastle disease is 3-4 titers. The antibody level higher than 3 log2 can only ensure that there are no massive deaths in clinical practice. However， when the antibody level is lower than 7 titers， the amount of eggs produced will be greatly reduced during the invasion of a strong virus strain. Therefore， HI antibody level of the breeder should be higher than 7 titers.
Feeding highquality fullprice feed to ensure good body constitution
Some farms feed the chickens with "lowprice feed"， which may reduce the resistance of chickens. Therefore， the breeders should establish the idea of "high input and high output" and feed the chickens with good quality feed to enhance body constitution and improve disease resistance. Wuzhen SU et al. Diagnosis and Etiological Analysis of Atypical Newcastle Disease in Mountain Forest Chickens in Huaihua City
References
[1] LI SS， LI JY， et al. Analysis of the causes of local epidemic of chicken Newcastle disease and its countermeasures[J]. Guide to Chinese Poultry， 2007 （1）：30.（in Chinese）
[2] WU D， GAO GZ， et al. Analysis of the causes and prevention of atypical Newcastle disease in hens[J]. Veterinary Orientation， 2007 （9）：27-28.（in Chinese）
[3] LU MY. Prevention of atypical Newcastle disease in chickens[J]. Sichuan Animal & Veterinary Sciences， 2007 （9）：56-57.（in Chinese）
[4] WANG RD， BAI J， MA HL. Epidemiology and preventing measure of Newcastle disease[J]. China Animal Husbandry & Veterinary Medicine， 2005 （4）： 60-62.（in Chinese）
[5] ZHOU RH， DING Y， et al. Cause analysis and prevention measures of immune failure against Newcastle disease[J]. Livestock and Poultry Industry， 2007 （6）： 37-38.（in Chinese）
[6] HU H. Characteristics of occurrence and prevalence of poultry diseases in recent year in Huaihua City， Hunan Province[J]. Guide to Chinese Poultry， 2006， 23（11）：24.（in Chinese）
[7] HAN DQ， WANG H. Diagnosis and prevention of Newcastle disease in blackbone chickens[J]. China Poultry， 2006 （10）：49.（in Chinese）